We recently purchased a Prairie Milkweed plant. I’ve noticed that it is infested with clusters of tiny black insects. I also see that there is honeydew on the plant.
Is there anything I can do to get rid of these without compromising any Monarch larvae or caterpillars?
Thanks for your advice
Thank you for contacting the Toronto Master Gardeners – and thank you for planting Prairie Milkweed (Asclepias sullivantii)!
The fact that you are noticing honeydew was a helpful factor to include in your description. I found this article from the University of Minnesota which describes the impact of aphids on milkweed as this sounds like what might be occurring on your plant. Oleander aphids are yellow in colour and feed exclusively on milkweed so it doesn’t sound like you have this this species; however aphids can come in a wide range of colours, including black. You can use a garden hose to spray off the aphids and this will also remove the honeydew which can lead to sooty mold on your plant. As you so rightly pointed out, this solution runs the risk of washing off tiny monarch caterpillars. Applying an insecticide is to be avoided for the same reason.
There are a variety of other options, though, that you can use. Better Homes and Gardens provides a list of environmentally friendly options, including handpicking and squashing them, and planting companion flowers to discourage aphids.
For reference in the future, just in case other pests come your way, Prairie Milkweed is host to a long list of insects found on the website Illinois Wildflowers, such as Milkweed Beetles and Milkweed Bugs. Monarch Butterfly Garden also provides a thorough summary of insects that find milkweed attractive.
As an aside and to finish, please note that all parts of milkweed plants are toxic to pets so if you have a dog, like I do, who enjoys nibbling the various leaves in your garden, please consider planting your milkweed in a location that your pets cannot access.